Centrifugal pumps are used handle a wide variety of fluids under a broad range of operational conditions. Centrifugal pumps that are capable of handing abrasive slurries at low flow rates while developing high total dynamic heads are desired for many industrial processes.
One type of pump that is used to deliver low flow at high heads is called a Partial Emissions (PE) Pump. The name partial emissions comes from a feature whereby flow from the impeller chamber is controlled by a discharge nozzle such that only a small percentage of the total casing volume leaves the pump in any single impeller revolution. This maintains a high process fluid rotational velocity vector and a small radial velocity vector. The high rotational velocity vector is what gives the partial emission pump a higher head coefficient than seen for a traditional centrifugal pump stage.
To meet higher application head, designers must increase the impeller tip speed or increase the number of stages. Both methods have been employed for clean liquid applications, but existing designs pose reliability and safety problems when handling erosive fluids. The high fluid relative velocities and relatively small passages of low Ns pumps result in erosive wear to pressure containing parts. This wear is exacerbated in non-linear fluid passages where additional energy is transferred to the fluid passage when the fluid is forced to change direction.
Two-stage PE pumps have been described wherein two coaxial impellers reside in separate chambers defined in part by an interstage body separating the two stages. Fluid flows between the first stage and the second stage, at a relatively high velocity, within an annular duct arranged in a series of turns within the main pump body such that fluid leaving the first stage impeller is turned through more than 180 degrees to position it for entry to the second stage impeller. Because the fluid ducts between the stages are, for the most part integral with the pressure containing components, any erosive wear will tend to decrease the pressure retaining capability of the pressure containing components.
One accepted practice for handling abrasive laden fluids at high velocities is to use wear liners that isolate the pressure containing components of the pump from the high velocity fluid, designed such that all of the high fluid velocity areas of the pump are encompassed by the internal liners. Wear occurs on the liner surfaces and not on the surfaces of the pressure retaining components. Lined pumps have been designed for both single and multistage applications. However, existing designs are of the conventional centrifugal pump design wherein design flow for a given impeller diameter and rotative speed is controlled by the first stage impeller design. This allows designers to utilize circumferential diffusers, guide vanes, return channels and the like, to channel flow from one stage to the next. These designs would be unsuitable for a PE pump, where the low flow rates would result in efficiency losses due to excess interstage diffusion.
Therefore it would be of use to have a multistage slurry pump of a partial emissions design where the pressure retaining parts are protected from erosive wear by liners within the pressure containing components of the pump, designed such that all of the high fluid velocity areas of the pump are encompassed by the liners. It would also be desirable to retain the partial emission design features and benefits in managing flow from the discharge of one stage to the inlet of the next.